The present invention relates to a propeller and method for manufacturing same, and more particularly, to a propeller manufactured using a lattice block material.
Propellers for ships are quite large, often spanning over 10 feet in diameter and are typically made of a bronze alloy. The propellers can be manufactured as a single casting or can be designed to have a plurality of blades cast separately that are then attached to a separate central hub, with the respective components being generally solid castings. The combination of size and material makes the propeller quite heavy and places great mechanical stresses on the blade attachment components, main shaft and main shaft bearings that must support the weight. The propeller can even be limited to a smaller diameter than desired for an application because the weight of a larger propeller imparts excessive stresses on such components. Large propellers solid cast propellers and propeller blades can also have material cross-section dimensions that are so large that the solidification of the material results in non-optimal through-section microstructure and reduced mechanical strength properties, such as tensile strength, yield strength, elongation and fatigue life.
In an attempt to address such shortcomings, and in particular, to reduce the weight of the propeller, the thickness of the blades has been reduced with respect to the chord length of the blades. This, however, can result in compromised cavitation performance and reduced mechanical strength properties of the propeller.
In addition, prior propellers have generally fixed modal/vibrational characteristics due to the material mass and properties of the propeller blades. Although such propellers can be balanced by the addition or removal of material to one or more of the blades, there are limitations to the manner such balancing can be performed while preserving the performance and structural integrity of the propeller.
A type of casting technology has been developed by the Jonathan Aerospace Materials Corporation of Wilmington, Mass. that creates a cast object having a continuous three-dimensional lattice of support spars with spaces between the spars being hollow, occasionally referred to as lattice block material or LBM. Such technology is disclosed, for instance, in International Patent Publication No. WO 99/55476, entitled “Method and Device for Casting Three-Dimensional Structured Objects”, published Nov. 4, 1999, the contents of which are incorporated by reference herein. To provide the device and method for casting three-dimensional structured objects which are economical and allow for the objects to be produced to be cast in a form in which they can easily be reused or produced, the invention provides for the device to comprise several cores (1; 31, 32) which each have essentially the form of a prism and at least three walls which are parallel to an axis or slightly convergent. The cores are constructed of known casting sand compositions. The prism shapes and cross-sections are chosen such that several cores (1; 31, 32) can be juxtaposed by their prism surfaces (2, 3, 4) in a substantially tight and space-filling manner. At least part of the prism surfaces (2, 3, 4) presents recesses or casting channels (6, 7, 8, 9) which form a continuous structure when the cores (1; 31, 32) are assembled. As regards the method, the hollow form is composed of several cores with a prism-shaped cross-section in such a way that the prism surfaces lie against each other in a substantially compact and flush manner and the cores substantially fully fill out the casting space provided for, with recesses in the prism surfaces defining the structure to be cast (the spars).